1. Field of the Invention
The present invention relates to a method of manufacturing an aluminum alloy sheet for use in body panel material for automobiles and the like, and to the aluminum alloy sheet manufactured by this method. More particularly, the present invention is concerned with an aluminum alloy sheet capable for recycling and excellent in formability such as deep drawing and bulging.
2. Description of the Related Art
Recently, for the purpose of environmental protection and reducing fuel consumption, light-weight structural materials have been demanded. In particular, endeavors to develop light-weight automobile parts, which have been conventionally formed of mild steel sheet, have been aggressively pursued. In an attempt to employ light-weight automobile parts, an aluminum alloy sheet has started to be used for automobile parts, such as automotive wheel parts, and structural materials such as constructional materials.
The aluminum alloy sheet used as a structural material is required to be excellent in properties including strength, formability, and corrosion resistance. For this reason, an Al--Mg alloy being well-balanced in the above-mentioned properties, is generally used.
However, the conventional aluminum alloy sheet is inferior in formability due to poor ductility compared to a mild steel sheet. The poor ductility is caused by the presence of a coarse intermetallic compound in the aluminum alloy sheet. Attempts have been made to improve the ductility by increasing the purity of the alloy metal matrix or subjecting an aluminum alloy, whose Mg content has been increased, to an annealing treatment at a high temperature so as to decrease the content of the coarse intermetallic compound. It is expected that any of these attempts inevitably increase manufacturing cost, causing significant problems when the attempts are put into practice.
An aluminum material is easily recyclable as well as light-weight. However, the recycling produces contamination with impurities, namely, elements other than the alloy elements. The coarse intermetallic compound derived from the impurities present in the alloy metal matrix decreases the ductility, leading to poor formability.
With increasing the constituent particles by recycling, precipitates and recrystallization are facilitated, with the result is that the grain size decreases. When the grain size of the aluminum alloy sheet decreases, ductility and formability deteriorate. Further, with decreasing grain size, the Ruders line frequently appears, affecting the appearance of the aluminum alloy sheet.
Then, in order to increase the grain size, a method is employed involving application of a cold rolling treatment to the aluminum alloy at a relatively small cold reduction to lower the driving force of the recrystallization. On the other hand, when the grain size is excessively large, ductility and formability also deteriorate, forming an orange peel on the aluminum alloy sheet. Accordingly, to realize a material excellent in ductility and formability having good appearance after sheet formation, it is necessary to select an appropriate cold reduction.